1. Field of the Invention
The present invention relates to an image process and, more particularly, to a region based illumination-normalization method and system.
2. Description of Corresponding Art
An image has two primary types of image information, illumination information and object information. During an image analysis or an image processing application (such as a face recognition system or a mechanical vision system), the illumination information in the image is usually removed first to eliminate data deviations caused by different lighting effects on objects, thus bringing lighting effects in the processed image to a minimum.
FIG. 1 is a schematic drawing of a face image obtained under different lighting conditions. It illustrates how different lighting can cause different image results. Therefore, in order to utilize a plurality of images in the face recognition system, an illumination compensation process must be performed to the plurality of images to bring the lighting effects to a minimum. Currently, there are two types of illumination compensation processes. One is implemented by utilizing a statistical approach, and the other one is implemented by utilizing a space field analysis process or a frequency field analysis process. These two types of illumination compensation processes will be explained hereinafter.
The statistical approach employs an averaging method to reduce image differences so that the images are presented evenly. The space field analysis process uses a filter to filter image features (F0) and illumination information (I0) from the testing image, and normalizes the illumination information (I0) to serve as illumination information (Ia) so that any effect caused by lighting factors are minimized. The illumination information (Ia) and the image features (F0) are combined to form an illumination normalized image with low light sensitivity. For example, U.S. Pat. No. 6,122,408 discloses such an image processing method by using a formula:x(x,y)=α(x,y)·Y(x,y)+β(x,y),wherein, x (x,y) is an input test image, Y(x,y) is an output illumination normalized image, α(x,y) is a multiplication factor and β(x,y) is an addition factor. Accordingly, the output illumination normalized image Y(x,y) can be obtained by:Y(x,y)=(x(x,y)−β(x,y))/α(x,y).Thus, this patent focuses on how to obtain the multiplication factor and the addition factor. However, obtaining the multiplication factor and the addition factor is not easy and requires a lot of processing time.
Therefore, it is desirable to provide a region based illumination-normalization method and system to mitigate and/or obviate the aforementioned problems.